It is known to reproduce images by photographic means using a photosensitive composition composed of a mixture of a polymerizable compound having an ethylenically unsaturated bond, a photopolymerization inhibitor. As described in U.S. Pat. Nos. 2,927,022, 2,902,356, and 3,870,524, since the above-described type of photosensitive composition upon photopolymerization by irradiation of light is hardened and insolubilized, by forming a proper film or layer of the photosensitive composition, light-exposing the film or layer through a desired negative, and removing unexposed portions only by a proper solvent (hereinafter referred to as "development"), desired hardened images of the photosensitive composition can be formed. Such a photosensitive composition is, as a mater of course, very useful for making printing plates, etc.
Also, since a polymerizable compound having an ethylenically unsaturated bond itself does not have sufficient photosensitivity, it has previously been proposed to add thereto a photopolymerization initiator for increasing the photosensitivity. Examples of such photopolymerization initiators include benzyl, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone, and 2-ethylanthraquinone.
In the past, however, when the above-described conventional-type photopolymerization initiators have been used, the photopolymerizable composition has shown low responsiveness to hardening. Furthermore, image exposure for forming images requires a long period of time. Accordingly, when precise images are to be formed, if the operation is accompanied by a slight vibration, images having good image quality are not reproduced. Furthermore, the radiation amount of light energy for light exposure must be increased, which results in the formation of large amounts of heat. There is also a problem with the formation of heat in that a film of the photosensitive composition may be deformed or the quality thereof may be adversely affected.
Also, with these photopolymerization initiators, the photopolymerizing faculty thereof by a light source having a visible light region of longer than 400 nm. is remarkably lower than the photopolymerizing faculty thereof by a light source having an ultraviolet region of shorter than 400 nm. Accordingly, with a photopolymerizable composition containing conventional-type the photopolymerization initiators, the application range has been greatly limited.
With regard to photopolymerization systems responsive to visible light, various attempts have been proposed. For example, U.S. Pat. No. 2,850,445 indicates that a certain types of photoreducible dyes such as rose bengal, eosin, and erythrosine have effective visible light respondence. Furthermore, there has been proposed as improved techniques, a composite initiation system composed of a dye and an amine (See Japanese Patent Publication No. 20189/69); a system composed of hexaaryl-bi-imidazole, a radical generating agent, and a dye (See Japanese Patent Publication No. 37377/70); a system composed of hexaaryl-bi-imidazole and p-dialkylaminobenzylidene ketone (See Japanese Patent Application (OPI) Nos. 2528/72 and 155292/79); a system composed of a 3-keto-substituted coumarin compound and an active halogen compound (See Japanese Patent Application (OPI) No. 15503/83); and a system composed of a substituted triazine and a merocyanine dye (See Japanese Patent Application (OPI) No. 151024/79). The term "OPI" as used herein indicates an "unexamined published application".
These techniques are certainly effective with visible light. However, the photosensitive speed thereof is still unsatisfactory and further improvements are desired.
Recently, a method of increasing the sensitivity to ultraviolet rays and a method of forming images using a laser have been investigated. Also, a UV projection exposure method for use in making printing plates, making a direct printing plate by laser, laser facsimile, and photography, have already been practically employed. High-sensitive photosensitive materials for these methods have been developed. However, the sensitivity of these photosensitive materials is still unsatisfactory.